Heteropolyacids are valuable chemical compounds which can be used as acid and/or oxygenate catalysts. The processes cited hereinafter, relate to a variety of examples of various heteropolyacids and/or the method(s) for producing them;
US2006052240 describes a supported catalyst comprising a support having supported thereon at least one member selected from the group consisting of heteropolyacids and heteropolyacid salts, in which the heteropolyacid and/or heteropolyacid salt is substantially present in a surface layer region of the support to a depth of 30% from the support surface.
U.S. Pat. No. 6,624,325 describes a catalyst for producing lower fatty acid esters through esterification of a lower aliphatic carboxylic acid with a lower olefin, comprising a heteropoly acid or a, salt thereof held on a carrier, and having a specific surface area of the catalyst, as measured by a BET method, of 65 m<2>/g-350 m<2>/g. A process for producing the catalyst and a process for producing a lower fatty acid ester by using the catalyst are also provided.
U.S. Pat. No. 5,227,141 describes a membrane catalytic reactor which comprises a heteropolyacid selected from the group consisting of 12-tungstophosphoric acid, 12-molybdophosphoric acid, 12-molybdotungstophosphoric acid, and 12-tungstosilicic acid, and polysulfone membrane is provided. This membrane catalytic reactor is applicable to vapor-phase dehydration, dehydrogenation, oxidation, and simultaneous separation of organic or inorganic materials, particularly vapor-phase dehydration of ethanol.
US2004/024918 describes a catalyst for use in producing a lower aliphatic carboxylic acid ester, wherein the catalyst is produced by a process comprising a step of contacting the catalyst with a gas containing at least one member selected from water, lower aliphatic carboxylic acids and lower aliphatic alcohols; a process for producing the catalyst; and a process for producing a lower aliphatic carboxylic acid ester using the catalyst. The document further describes a siliceous support for use in a catalyst, which has a silicon content of from 39.7 to 46.3% by mass or a silicon content of from 85 to 99% by mass in terms of silicon dioxide or a crush strength of 30 N or more. By the use of a catalyst comprising the support, a lower aliphatic carboxylic acid ester is produced from lower olefin and a lower aliphatic carboxylic acid in a gas phase without causing great reduction of catalytic activity or cracking or abrasion of the catalyst.
Alkene(s) have traditionally been produced by steam or catalytic cracking of hydrocarbons. However, inevitably as oil resources are decreasing, the price of oil is increasing; making light alkene production a costly process. Thus there is an ever-growing need for non-petroleum routes to produce C2 and C2+ alkene(s), particularly ethene and propene, as these make useful starting materials for a number chemical processes, including the production of polymeric compounds, such as, polyethylene and polypropylene.
In recent years the search for alternative starting materials for C2 and C2+ alkene(s), production has led to the use of oxygenates such as alcohols (e.g. methanol, ethanol and higher alcohols). These alcohols provide a particularly attractive route for the production of alkene(s) since the said alcohols may be produced by the fermentation of, for example, sugars and/or cellulosic materials.
Alternatively, alcohols may be produced froth synthesis gas. Synthesis gas refers to a combination of hydrogen and carbon oxides produced in a synthesis gas plant from a carbon source such as natural gas, petroleum liquids, biomass and carbonaceous materials including coal, recycled plastics, municipal wastes, or any organic material. Thus, alcohol and alcohol derivatives may provide non-petroleum based routes for the production of alkene(s) and other related hydrocarbons.
Generally, the production of alcohols, for example methanol, takes place via three process steps: synthesis gas preparation, methanol synthesis, and methanol purification. In the synthesis gas preparation step, an additional stage maybe employed whereby the feedstock is first treated, e.g. the feedstock is purified to remove sulphur and other potential catalyst poisons prior to being converted into synthesis gas. This treatment can also be conducted after syngas preparation; for example, where coal or biomass is employed.
The processes cited hereinafter, relate to examples and alternative processes for the dehydration of an oxygenate feedstock; U.S. Pat. No. 4,543,435 discloses a process for converting an oxygenate feedstock comprising methanol, dimethyl ether or the like in an oxygenate conversion reactor into liquid hydrocarbons comprising C2-C4 alkenes and C5+ hydrocarbons. The C2-C4 alkenes are compressed to recover an ethylene-rich gas. The ethylene-rich gas is recycled to the oxygenate conversion reactor.
U.S. Pat. No. 4,076,761 discloses a process for converting oxygenates to gasoline with the return of a hydrogen-rich gaseous product to a synthesis gas plant or the oxygenate conversion reaction zone.
U.S. Pat. No. 5,177,114 discloses a process for the conversion of natural gas to gasoline grade liquid hydrocarbons and/or alkenes by converting the natural gas to a synthesis gas, and converting the synthesis gas to crude methanol and/or dimethyl ether and further converting the crude methanol/dimethyl ether to gasoline and alkenes. International Patent Application No. 93/13013 to Kvisle et al. relates to an improved method for producing a silicon-alumino-phosphate catalyst which is more stable to deactivation by coking. The patent discloses that after a period of time, all such catalysts used to convert methanol to olefin(s) (MTO) lose the active ability to convert methanol to hydrocarbons primarily because the microporous crystal structure is coked; that is, filled up with low volatility carbonaceous compounds which block the pore structure. The carbonaceous compounds can be removed by conventional methods such as combustion in air.
EPO publication No. 0 407 038A1 describes a method for producing dialkyl ethers comprising feeding a stream containing an alkyl alcohol to a distillation column reactor into a feed zone, contacting the stream with a fixed bed solid acidic catalytic distillation structure to form the corresponding dialkyl ether and water, and concurrently fractionating the ether product from the water and unreacted materials.
U.S. Pat. No. 5,817,906 describes a process for producing light alkenes from a crude oxygenate feedstock comprising alcohol and water. The process employs two reaction stages. Firstly, the alcohol is converted using reaction with distillation to an ether. The ether is then subsequently passed to an oxygenate conversion zone containing a metal aluminosilicate catalyst to produce a light olefin stream.
However, a major disadvantage associated with the typical dehydration methods used for producing alkene(s) from oxygenates, is that aromatic and alkane by-products are usually co-produced together with the targeted alkene products, and prove both difficult and expensive to separate, e.g. to recover ethane from ethane is both complex and expensive. For example, the well known methanol to olefin(s)—MTO—process can be described as the dehydrative coupling of methanol to alkenes and is a well known chemistry that can be employed to produce alkenes from alcohol(s). This mechanism is thought to proceed via a coupling of C1 fragments generated by the acid catalysed dehydration of methanol, possibly via a methyloxonium intermediate. Again, the main disadvantage of the said MTO process is that a range of alkenes are co-produced together with a range of various aromatic and alkane by-products, which in turn makes it very difficult and expensive to recover the desired alkenes.
Molecular sieves such as the microporous crystalline zeolite and non-zeolitic catalysts, particularly silicoaluminophosphates (SAPO), are known to promote the conversion of oxygenates by methanol to olefin (MTO) chemistry to hydrocarbon mixtures. Numerous patents describe this process for various types of these catalysts: U.S. Pat. Nos. 3,928,483, 4,025,575, 4,252,479 (Chang et al.); U.S. Pat No. 4,496,786 (Santilli et al.); U.S. Pat No. 4,547,616 (Avidan at al.); U.S. Pat No. 4,677,243 (Kaiser); U.S. Pat No. 4,843,183 (Inui); U.S. Pat. No. 4,499,314 (Seddon et al.); U.S. Pat No. 4,447,669 (Harmon et al.); U.S. Pat No. 5,095,163 (Barger); U.S. Pat No. 5,191,141 (Barger); U.S. Pat No. 5,126,308 (Barger); U.S. Pat No. 4,973,792 (Lewis); and U.S. Pat No. 4,861,938 (Lewis).
Additionally, this reaction has a high activation energy step—possibly in the methanol or dimethylether production step—hence to achieve high conversion there is a need for high temperatures, e.g. 450° C., to drive the reactions forward. Conventionally, various means, such as, a heated catalyst recycle and/or “Downtherm” heating systems, have been implemented in such systems in order to obtain such high temperature conditions. Unfortunately operating at these said high temperatures leads to major problems, such as, catalyst deactivation, coking and by-product formation. In order to avoid these problems the reactions may be operated at lower temperatures, but this necessitates an expensive recycle of intermediates and reactants.
These and other disadvantages of the prior art show that there is a clear need, within the field, for an improved and/or alternative process for the production of C2 and C2+ alkene(s) from oxygenates.